In recent years, concerning a driving scheme for vehicles such as a railroad vehicle, according to the development of a power electronics technique, a scheme for controlling an AC motor using an inverter that converts DC power into AC power is the mainstream.
However, when inverter control is performed, there is a problem in that electromagnetic noise occurs. This is caused by performing PWM (Pulse Width Modulation) control on an inverter when the DC power is converted into the AC power. In the PWM control, a pulse voltage used for ON/OFF control of a switching element of the inverter is a rectangular wave. A steep voltage change occurs at a rising edge and a falling edge of the rectangular wave.
Because of the steep voltage change, harmonics are always superimposed on an output current of the inverter with respect to a fundamental wave. The fundamental wave refers to an AC component of a frequency desired to be output to the AC motor. When an alternating current obtained by combining the fundamental wave and the harmonics is energized to the AC motor, physical vibration due to a frequency component of an electric current occurs in a stator and a rotor of the AC motor. Electromagnetic noise (direct noise) depending on a vibration frequency of the physical vibration occurs.
As the electromagnetic noise that occurs in the AC motor, besides the direct noise, there is also solid propagation noise that occurs because the vibration of the AC motor is propagated through a motor setting table such as a truck, to which the AC motor is attached, and a vehicle main body.